Light emitting diode (LED) lighting systems are becoming more prevalent as replacements for existing lighting systems. LEDs are an example of solid state lighting (SSL) and have advantages over traditional lighting solutions such as incandescent and fluorescent lighting because they use less energy, are more durable, operate longer, can be combined in multi-color arrays that can be controlled to deliver virtually any color light, and contain no lead or mercury. In many applications, one or more LED chips (or dies) are mounted within an LED package or on an LED module, and such a device may make up part of a lighting unit, lamp, “light bulb” or more simply a “bulb,” which includes one or more power supplies to power the LEDs. An LED bulb may be made with a form factor that allows it to replace a standard threaded incandescent bulb, or any of various types of fluorescent lamps.
Color reproduction can be an important characteristic of any type of artificial lighting, including LED lighting. Color reproduction is typically measured using the color rendering index (CRI). The CRI is a relative measurement of how the color rendition of an illumination system compares to that of a theoretical blackbody radiator. In practical terms, the CRI is a relative measure of the shift in surface color of an object when lit by a particular lamp. The CRI equals 100 if the color coordinates of a set of test surfaces being illuminated by the lamp are the same as the coordinates of the same test surfaces being irradiated by the theoretical blackbody radiator. Daylight has the highest CRI (100), with incandescent bulbs being relatively close (about 95), and fluorescent lighting being less accurate (70-85). With multi-chip LED devices, the color of the light from the device will be affected by the color of light emitted by each chip in the device. Chips must be mixed appropriately to maintain the desired color parameters for a device, including the CRI.